Field of the Disclosure
The present disclosure generally relates to image forming and, more particularly, to an image forming apparatus using an electrophotographic method such as a printer, a copying machine, or the like.
Description of the Related Art
To obtain a color image with high image quality, it is generally important to output color components including yellow (Y), magenta (M), cyan (C), and black (Bk) such that the density of each color component is properly controlled in forming the color image. In view of the above, in color image forming apparatuses using the electrophotographic method, it is known to use an image density control technique to obtain an output image with color components with stably controlled density. In the image density control technique, a toner image called a patch is experimentally formed on an image bearing member, and the density of the toner image (the toner bearing amount) is detected using a density sensor and fed back to an image formation condition such as a peripheral speed of a development roller, or the like (see, for example, Japanese Patent Laid-Open No. 08-146749).
The density sensor is generally realized using a combination of a light emitting device such as an LED and a photodetector such as a photodiode or a cadmium sulfide cell (CdS). A surface of an intermediate transfer belt or the like to be measured is illuminated with light from the light emitting device, and specular reflection light from the surface of the intermediate transfer belt is detected by there by the photodetector. When toner is put on the intermediate transfer belt, a reduction in intensity of specular reflection light occurs depending on an amount of toner put thereon (hereinafter, referred to as a toner bearing amount), and this change in light intensity is detected by the density sensor and output from the density sensor. FIG. 7A is a graph in which a vertical axis represents the output of the density sensor and a horizontal axis represents the toner bearing amount. As shown in FIG. 7A, the output from the density sensor tends to decrease as the toner bearing amount (the toner density) increases. Therefore, when an image used as the patch in the image density control is an image of a type that consumes a large toner bearing amount (hereinafter such an image will be referred to as a solid image), the output has a small change in response to a change in the toner bearing amount, which makes it difficult to accurately detect the toner bearing amount. To avoid the above situation, instead of a solid image, an image that is small in toner bearing amount and great in change in output compared to the solid image is used. In the conventional image density control, a change in density of a solid image is estimated from a change in output for an image that consumes smaller in toner bearing amount than is consumed by the solid image, and the density of the solid image is controlled based on the estimation. However, depending on the condition of using toner, the density obtained after the control is not necessarily what is expected. In particular, when the toner bearing amount is greater than is necessary, the following problems may occur.
FIG. 7B illustrates an example of a change in chromaticity that may occur when the bearing amount of cyan toner is increased. In this figure, chromaticity is plotted in an a*-b* plane. The toner bearing amount increases in a direction denoted by a thick solid arrow in FIG. 7B. In FIG. 7B, an optimum toner bearing amount is obtained near a point denoted by α, and this toner bearing amount usually provides a maximum density. As may be seen from FIG. 7B, when a greater amount of toner than is necessary is borne beyond the point α, a great change occurs in hue angle θ. In other words, in FIG. 7B, plotted points are on a solid line of a hue angle θ1 as far as the bearing amount of tone is within a proper range, but once the bearing amount of tone increases beyond the point α, plotted points are on a dot line of a hue angle θ2 different from θ1. The occurrence of the large change in hue angle θ indicates an occurrence of an abrupt change in tint near the maximum density. A possible reason for such a change in tint is a nonuniform distribution of a colorant within toner, and thus a resultant nonuniform distribution of the colorant on a fixed image. When the toner bearing amount is small, the nonuniform distribution of the colorant does not have a significant influence. However, when the toner bearing amount is excessive the nonuniform distribution of the colorant may cause a change in a tint. In image processing performed by a color image forming apparatus, it is generally assumed that a change in tint occurs gradually and monotonically. Therefore, a large change in tint may result in a deviation of a color balance in an output image, which may cause not only a reduction in image quality but also an increase in consumption of toner.